1. Field of the Invention
The present invention relates to a light beam scanning device that performs scanning with a light beam from a light source while detecting the light beam, an image forming apparatus, and a light beam scanning method.
2. Description of the Related Art
In general, an image forming apparatus employing a light beam scanning device modulates a light beam by image data, deflects the light beam at constant angular velocity in the main scanning direction by rotating a polygon mirror, and corrects the deflection of constant angular velocity to deflection of constant linear velocity, thereby scanning the surface of a photosensitive body.
However, with respect to the conventional image forming apparatus, there is a problem in that each mass-produced product has different image magnification because of variations in the characteristics of the lens of its light beam scanning device. In particular, in the case of employing a plastic lens, it is very likely that the shape and the refractive index of the plastic lens vary because of variations in ambient temperature or the internal temperature of the apparatus. When such variations in the shape and the refractive index occur, the scanning position on the image surface of the photosensitive body changes, thus preventing a high-quality image from being produced.
Further, in image forming apparatuses employing multiple laser beams and corresponding lenses to form a multicolor image, color misregistration occurs because of their respective magnification errors, so that a high-quality image may not be produced. Accordingly, it is preferable to equalize the image magnifications of the multiple colors.
Therefore, with respect to image forming apparatuses that form images by performing light beam scanning, Japanese Laid-Open Patent Application No. 2001-66524 (Document 1) discloses a technique for correcting image magnification error in the main scanning direction caused by various factors such as variations in ambient temperature and apparatus internal temperature.
According to the technique of Document 1, the time difference between detection of a light beam by first light beam detection means and detection of the light beam by second light beam detection means is calculated (measured) based on the number of counted cycles of a certain clock signal and the relationship in phase between clock signals and light beam detection signals, thereby improving correction accuracy without the necessity of a high-speed clock signal.
Japanese Laid-Open Patent Application No. 2002-162586 (Document 2) discloses an apparatus using multiple light beams in which correction is performed so that the light beams are equalized in intensity when the light beams enter a synchronization detection sensor.
According to the technique disclosed in Japanese Laid-Open Patent Application No. 5-96778 (Document 3), the area other than the image area corresponding to recording paper in the main scanning direction is illuminated with a light beam of constant intensity.
In the conventional image forming apparatus, the amount of light on the photosensitive body may vary depending on a scanning position in the main scanning direction because of the angle of incidence of a light beam to the surface of the polygon mirror, variations in lens transmittance, and variations in mirror reflectance. If such variations in the amount of light on the photosensitive body are caused by the scanning position, variations in image density occur even if an image is formed with a constant amount of light of a light beam. As a result, a high-quality image is prevented from being produced.
In image forming apparatuses using multiple light beams and corresponding lenses to form a multicolor image, images of different colors are superposed. Therefore, production of a high-quality image may be prevented by color unevenness due to variations in the amount of light in each color. Accordingly, it is preferable to equalize the image magnifications of the colors as much as possible.
In this regard, Japanese Laid-Open Patent Application No. 6-98104 (Document 4) discloses a technique for improving image quality by correcting the shading of or variations in the amount of light in the main scanning direction.
According to this technique, the improvement of image quality is achieved by shading correction that controls an LD driver IC based on the shading of light power within a premeasured scan width so that the amount of light on the surface of a photosensitive body is constant.
FIG. 1 is a waveform chart illustrating the output signal of one of the light beam detection means of Document 1.
Such an output signal is generated by converting the output of a PD (photodiode) to a binary value based on a fixed threshold level. However, a variation in the amount of light at the time of measurement may cause a variation in the output of the PD, so that the output timing of the light beam detection signal may also change accordingly. Referring to FIG. 1, the output signal may change from [1] to [2] as the amount of light changes from [1′] (large) to [2′] (small).
This change of the output timing of the light beam detection signal causes the measured time difference to change, thus causing a problem in that magnification error is prevented from being corrected with accuracy. That is, Document 1 does not go so far as to consider making constant the condition of the amount of light at the time of measurement.
According to Document 2, with respect to the above-described problem, no equality in beam intensity is required between the light beams, and it is necessary to equalize the beam intensity of one of the light beams at the time of measuring the time difference. That is, Document 2 does not go so far as to consider making a constant condition at the time of measurement with respect to one of the light beams.
According to Document 3, light beam intensity is constant with respect to the area other than the image area. Considering the case of performing shading correction as performed in Document 4, for instance, an attempt to make constant the light beam intensity in the area other than the image area, where an image is formed, may make it difficult to return the amount of light to a desired value in the image area.
A description is given of the reason why such a problem occurs.
Consideration is given to the case where shading correction as performed in Document 4 is performed in an ideal state where the number of points at which the amount of light is corrected is equal to the total number of dots within the area subjected to the correction. In this case, if the amount of light on a photosensitive body varies in the main scanning direction as illustrated in FIG. 2A, the amount of light can be corrected to be constant in the main scanning direction as illustrated in FIG. 2C by a correction signal as illustrated in FIG. 2B.
However, providing a large number of light amount correction points as described above increases a control circuit in scale. Accordingly, in practice, the amount of light is corrected section by section (area by area). In the case of performing such correction, the varying signal as illustrated above in FIG. 2A, for instance, is corrected by a correction signal as illustrated in FIG. 3A. As a result, the amount of light varies significantly at points where the level of the correction signal switches as illustrated in FIG. 3B, thus resulting in degraded image quality.
Therefore, in general, an RC filter is inserted into the correction signal to reduce variations in the correction signal so that the correction signal approximates the state of FIG. 2B, thereby preventing a sudden change in the amount of light (reducing the difference in level of the amount of light). This difference can be made smaller by increasing the filter constant, so that image quality can be increased. However, in the case of changing the amount of light greatly, the change cannot be followed, thus causing a problem in that it takes time before the amount of light reaches an actual set value.
Therefore, it may be difficult for the technique of Document 3 to return the amount of light immediately to a desired value at a position where the area other than the image area switches to the image area. However, when consideration is given to making constant the amount of light at the time of measurement in order to perform the above-described magnification error correction with accuracy, it is only at the time of measuring the time difference that the amount of light is required to be constant.
Further, the first and second light beam detection means should not always have to be equal in beam intensity. There should be no problem even if the first and second light beam detection means are different in beam intensity. The condition of the amount of light should only have to be constant at a measurement part.